This invention relates to powder spray coating and, in particular, to the recovery of powder which has not adhered to the article to be coated from a powder spray booth.
The powder which does not adhere to an object to be coated in a powder spray booth is known as the xe2x80x9coversprayxe2x80x9d. Overspray powder may miss the object to be coated, rebound from a surface of the object, or be deflected by the electrostatic fields in the spray booth away from the object.
It is known to recover overspray powder by exhausting the air from the booth and hence the air-borne overspray. Filter elements have been used to recover overspray from powder spray booths and are capable of removing substantially all the overspray from the spray booth exhaust air so that the powder may be reused. Such filter elements require frequent cleaning in order to prevent them from becoming blocked or clogged with powder and this is usually carried out by reverse pulses of pressurised air which dislodge powder adhering to the filter elements.
It is also known to use cyclone separators instead of, or in conjunction with, filter elements for collection of overspray. Cyclone separators are effective in removing the majority of the overspray from the spray booth exhaust air and are easier to clean than filters because very little powder is deposited inside the cyclone housing.
In known arrangements, the cyclone separators used in powder recovery have either been very large single or dual side by side cyclones or multi cyclones having tangential or axial feed inlets. In cyclones of the vertical feed type, the air entrained overspray powder is fed vertically downwards from a common inlet manifold into the separators and a circumferential velocity is imparted thereto by a number of radial veins. In such vertical cyclones, the overspray powder is separated from the air by the combined effect of centrifugal and gravitational forces and falls to the bottom of the separators there to be collected and removed. The cleaned air is then directed vertically upwards through ducts, one per cyclone separator, which pass through the centre of the veins and into an exhaust manifold. The cleaned air then passes through a further powder recovery unit containing a number of filter elements to remove any fine powder particles entrained in the air.
Such vertical feed cyclone separators, when arranged in a module of a cluster of small cyclones, render the module difficult to clean thoroughly as power tends to impinge and adhere to the inlet areas of the cyclone and the veins directly below the inlet areas and to the vertical exhaust ducts passing through the centre of the veins. These surfaces are difficult to clean properly, particularly where there is a large number of cyclone separators because these areas are generally not easily accessible. The cleaning of such a powder recovery module, when changing the colour of powder to be recovered thereby, for example, is a lengthy process.
An arrangement intended to reduce the amount of overspray retained in the cyclone separators of a recovery module and facilitate cleaning of the retained overspray is disclosed in European Patent Application Number 94929600.8. A powder recovery module is provided comprising at least two tangential feed cyclone separators, each with a horizontal tapered feed channel having a dedicated inlet, the inlet lying in a common plane disposed near the ceiling of the powder spray booth when the module is mounted to one of the side walls of the powder spray booth. An overspray intake is located near the floor of the powder spray booth and a channel connects the intake to the inlet. The arrangement has the advantage that a greater proportion of overspray is separated from the spray booth exhaust air through use of the tangential feed cyclone separators in combination with the tapering feed channels. The geometrical arrangement of the inlets to the cyclone separator feed channels and the configuration of the tangential feed cyclone separators facilitates the cleaning of the surfaces to which overspray may adhere by means of an air hose, for example.
The above described arrangement has been found to work well in practice. However, changing the powder colour may still be a time-consuming task because of the time required to clean the booth. The booth needs sweeping to remove powder which was deposited on the walls and ceiling and on the floor and cannot be extracted by air exhaustion. In addition the cyclone separators and the channels leading thereto need to be cleaned which can be a time-consuming and difficult operation because of the limited access to the interiors thereof.
Proper cleaning of the walls and floor of a powder spray booth between colour changes is very important to ensure acceptable production quality. Such cleaning may be carried out manually by an operator using a scraping and/or compressed air jet apparatus. Booths have been proposed which are intended to automate the deposited overspray powder cleaning process. US Pat. No. 4153008 discloses a booth with a floor in the form of a conveyor belt which continuously transports powder to the exterior of the booth where the powder is removed from the belt by a suction nozzle and then passed to a separator. US Pat. No. 4430956 discloses a booth with a rotating arm on the floor which sweeps up deposited powder and passes it to a hopper positioned below the floor. European Patent Application No. 0701868 also discloses an arrangement with a floor scraper but in this case scrapers are also provided for the walls and ceiling, the collected powder again being transferred to a collection hopper located below the booth.
Even with the relatively sophisticated and therefore costly arrangements proposed for cleaning of deposited powder, the cleaning times for each change of powder colour are still considerable with consequently long periods during which the booth is out of operation. Furthermore, effective cleaning is still not guaranteed because of the difficulties involved in total removal of the powder which may be very fine.
One solution involves the use of a number of booths each dedicated to spraying of a particular powder colour. In an alternative solution, two or more booths are provided which can be moved alternately in and out of the production line to allow cleaning of one whilst the other is operational and vice versa.
British Patent Application No. 2141642 proposes that the deposited powder cleaning problems discussed above should be overcome by use of a booth in which the roof and/or walls are provided with replaceable coverings. The booth includes a collapsible suction tube for removal of air-borne overspray and also has a floor in the form of a rotatable disc by means of which deposited overspray passes outside the booth to a suction collector which is also connected to the collapsible suction tube. The proposed arrangement like the others discussed above suffers from the disadvantage that it is complicated and therefore costly and prone to failure.
German Utility Model G8907538.2 describes a powder spray booth with a primary extraction system comprising a filter around the spraying area and a secondary extraction system comprising extraction channels near the booth floor through which deposited overspray is sucked. The two extraction systems have separate suction devices.
It is an object of the present invention to provide a colour spray apparatus in which powder colour can be changed quickly and easily and which, moreover, is of relatively simple construction.
It is another object of the present invention to provide a colour spray apparatus in which both air-borne and deposited overspray can be recovered by a system involving a small number of relatively simple parts which are themselves readily cleaned.
It is a further object of the present invention to provide a powder recovery system for recovering both air-borne and deposited overspray employing suction means in which the amount of suction applied to the air-borne overspray and the deposited overspray can be varied.
It is a still further object of the present invention to provide a powder recovery module including at least one cyclone separator in which sieving to remove foreign objects or powder agglomerations can be carried out simply and efficiently and without impairing the operation of the cyclone separator.
A powder spray apparatus, in accordance with one aspect of the invention, comprises a powder spray booth having a floor, side walls and end walls, one or more powder spray devices positioned with respect to the side walls to spray articles passed through the booth via access openings in the end walls, a powder recovery module comprising at least one cyclone separator having an intake disposed close to, or towards, the ceiling of the powder spray powder spray a pin take at each end of the booth, each overspray intake being positioned close or adjacent, to the floor of the powder spray booth and the end wall and a feed channel connecting each intake with the cyclone separator, and a scraper bar for continuously reciprocating across the booth floor between the end walls to collect powder on the booth floor and move collected powder towards the end walls of the spray booth adjacent to which the overspray intakes are provided.
The floor of the booth is swept clean by the scraper bar which reciprocates backwards and forwards to gather up overspray and deliver it at the end of each stroke to one of the intakes. The collected powder is drawn into an air stream and, via a low level exhaust vent, to the cyclone separator. The location of the cyclone separator intakes, adjacent the end walls, in combination with use of a scraper bar has been found to result in an efficient removal of overspray powder from the booth.
In a preferred embodiment, each feed channel includes a vertical section between the respective intakes and the inlet of the cyclone separator, the vertical section being formed in two parts, one of which is removable such as to provide access to the interior of the vertical section. The arrangement facilitates cleaning of the powder recovery module by providing for access to the powder passage from the booth to the cyclone separator or separators.
A powder spray apparatus in accordance with another aspect of the invention comprises a powder spray booth having a floor, side walls and end walls, one or more powder spray devices positioned with respect to the side walls to spray articles passed through the booth via openings in the end walls and a powder recovery module comprising separation means for separating powder from a powder-air mixture, suction means for removing overspray powder from the booth, an exhaust duct connected to the suction means and communicating with the booth a passage of air-borne overspray powder, collection means for collecting deposited overspray powder from the booth floor and a feed channel connected to the suction means, wherein the collection means moves the collected deposited overspray powder to the feed channel through which it is passed under the action of the suction means to the separation means characterised in that the exhaust duct forms part of the feed channel and in that the apparatus includes means for adjusting the relative amount of suction provided by the suction means to, respectively, the air-borne overspray and the deposited overspray.
The dual removal both of air-borne overspray and deposited overspray increases overall usage efficiency. However, by making at least one part of the removal systems common to both, the recovery module is simplified as to its construction and cleaning. During a spraying operation greater suction can be provided to the air-borne overspray, as the amount thereof will likely be greater than the amount of deposited overspray. At or near the end of a spraying operation, the amounts of suction can be varied so that the suction on the deposited overspray is increased corresponding to the decrease and eventual cessation of air-borne overspray so that the maximum suction can be applied to the deposited overspray to thereby facilitate and accelerate cleaning of the booth.
The exhaust duct may comprise a pipe with an aperture providing for communication between the booth interior and the pipe interior. The aperture may be defined by a cover positioned across an opening in the pipe but spaced from the pipe. The relative amount of suction can be varied in a simple fashion by adjusting the position of the cover relative to the pipe opening thereby to vary the size of the aperture.
Very preferably, the powder recovery module comprises two cyclone separators, one adjacent each end wall of the powder spray booth.
The powder recovery module may comprise a filter module for removal of fine powder particles entrained in the exhaust air. Suitably, for reasons of economy and space, the two cyclone separators are connected to a common filter module.
The scraper bar may be magnetically coupled to a drive positioned externally of the powder booth. The advantage of this is that the drive mechanism is protected from the powder and the amount of equipment which needs to be cleaned on changing powder colour is reduced.
To maximise the amount of powder which settles on the floor of the booth and can, therefore, be collected by the scraper bar, the booth walls and ceiling are preferably formed from non-conductive material. For example, the booth ceiling may be made from a plastic material as may be the cyclones and channels which connect these to the booth. Alternative materials include stainless steel.
A further advantage of constructing the booth from non-conductive material is that the amount of overspray powder which is passed to the cyclone is relatively constant during a spraying operation. If, instead, powder is allowed to build up on the booth space and walls, then, on cleaning down, large quantities of powder may be delivered to the cyclones which will result in cyclone efficiency drop off.
The invention in a further aspect provides a method of operating a powder spray apparatus having a powder spray booth including at least one powder spray device for spraying articles passed through the booth, the method comprising exhausting air from the booth by suction at a first extraction location to withdraw air borne overspray powder via the first extraction location, collecting deposited overspray powder from the interior of the booth, transferring the collected deposited overspray powder to a second extraction location, withdrawing the collected deposited overspray powder in an air current by suction at the second extraction location, and separating the overspray powder from the air wherein the method includes varying the relative amounts of suction applied at the first and second extraction locations.
As discussed above, the advantage of this is that the overspray powder extraction can be accomplished in a particularly efficient manner, facilitating and accelerating booth cleaning therefore changing of the colour of powder being sprayed.
Suitably the or each cyclone separator is provided with a transfer means for transferring powder from the cyclone separator to a powder collection hopper through a sieve located between the cyclone separator and the powder collection hopper. The location of the sieve is outside the cyclone with the separated powder being removed from the cyclone before being deposited onto the sieve means that the cyclone design and performance is not limited by the need to accommodate a sieve within its body.
The transfer means may comprise a venturi pump. A mini cyclone separator may be located between the transfer means and the sieve to remove the transfer air and enhance sieve efficiency.
The collection hopper and sieve, as well as the mini cyclone if provided, may be located in a ventilated enclosure. This allows the sieve to be cleaned without contaminating the workshop.